Fast relay turn on circuit with low holding current

ABSTRACT

A drive circuit responsive to an applied electrical signal for generating a voltage to control the actuation of an electromagnetic relay is disclosed. The voltage has a first and a second level for picking-up and holding-in, respectively, the relay. The first voltage level has a peak value which is developed by an internal charging circuit and is additive to the second voltage level supplied by an external source. The first voltage level is higher than required to actuate the relay which enhances the pick-up speed of the relay.

BACKGROUND OF THE INVENTION

This invention relates to a relay drive circuit, and more particularly,to a relay drive circuit for an electromagnetic relay.

Electromagnetic relays are typically actuated or picked-up within aspecified time duration, commonly termed the "pick-up" or "operate" timeof the relays, by the application of a corresponding D.C. voltage acrosstheir energizing coil. The pick-up time of the relay decreases withincreases in the level of the applied D.C. voltage. After the pick-uptime of the relay has expired, the level of the applied D.C. voltage maybe decreased and the relay will maintain or hold-in its actuated state.

In electrical systems, such as high-voltage distribution networks,electromagnetic relays are commonly used to interconnect varioussubsystems within the network. The various subsystems typically have anelectronic circuit in their output stage which controls the applicationof a D.C. voltage for actuating an electromagnetic relay. The electroniccircuit is commonly called "a relay drive circuit", and controls theapplication of the D.C. voltage to the relay typically developed fromthe power supply source of the subsystem.

The speed of response of the output stage of the subsystem isessentially controlled by the pick-up time of the electromagnetic relay.It is desirable that the subsystem have a relatively fast speed ofresponse; however, electomagnetic relays typically have relatively longpick-up times. For example, the application of a D.C. voltage such as 18V. D.C. may typically produce a relay pick-up time of 26 milliseconds.The pick-up time of the relay may be reduced by increasing the D.C.voltage level of the power supply source of the subsystem which in turnincreases the D.C. voltage level generated by the relay drive circuit.Increasing the D.C. voltage level generated by the relay drive circuitin turn increases the D.C. voltage applied across the energizing coil tothereby increase the speed of response of the subsystem. It is desirablenot to supply the large energizing voltage on a steady state basis.However, the overall electronic circuitry within the subsystem,excluding the electromagnetic relay, operates satisfactorily withoutincreased D.C. voltage levels of the power supply. Therefore, it isconsidered undesirable to impose an overall increase to the subsystempower supply requirements to accommodate the needs of one particularuser that being an electromagnetic relay that will be held in with thesame D.C. voltage supplied.

Accordingly, it is an object of the present invention to provide a relaydrive circuit which enhances the speed of response of theelectromagnetic relay without causing any increase to the power supplyrequirements of the subsystem.

It is another object of the present invention to provide means wherebythe D.C. level of energizing the operating signals for theelectromagnetic relay controlled by the relay drive circuit may beadaptable to the requirements, such as pick-up and hold-in voltages, ofthe electromagnetic relay.

These and other objects of the invention will become apparent to thoseskilled in the art upon consideration of the following description ofthe invention.

SUMMARY OF THE INVENTION

The present invention is directed to a drive circuit responsive to anapplied electrical signal for controlling the actuation of a relayhaving an energizing coil with a first and a second end arranged acrossan output stage of the circuit. The circuit is adapted to be energizedby a D.C. voltage present between a positive and a negative potential.The circuit comprises a charging circuit, a second and a third diode, afirst and a second switching means, and means for dissipating inductivestored energy of the energizing coil. The charging circuit comprises aserially-arranged first resistor, a capacitor and a first diode. Thefirst resistor has a first end connected to the positive potential. Thefirst diode has its cathode connected to the negative potential. Thesecond and third diodes have their cathodes connected together. Thesecond diode has its anode connected to a second end of the firstresistor and to a first end of the capacitor. The third diode has itsanode connected to the first end of the energizing coil. The second endof the energizing coil is connected to the positive potential. The firstswitching means has first, second and third electrodes. The firstelectrode is connected to the negative potential. The second electrodeis adapted to receive the applied electrical signal. The third electrodeis coupled to the connected cathodes of the second and thirddiodecapacitor to be charged to a voltage substantially equal to theD.C. voltage, and (ii) upon the occurrence of said applied electricalsignal it is rendered conductive and supplies a path to forward-bias thesecond diode which provides a path to initiate a discharge of thecapacitor. The second switching means has first, second, and thirdelectrodes. The first electrode is connected to a second end of thecapacitor. The second electrode is coupled by a second resistor to thenegative potential. The third electrode is connected to the first end ofthe energizing coil. The seconcapacitor. The means for dissipatinginductive stored energy is coupled to the energizing coil. The chargingcircuit, the second and third diodes, and the first and second switchingmeans are arranged so that upon the occurrence of the applied electricalsignal, both said first and second switching means are renderedconductive and an initial voltage equal to the sum of the chargedpotential of the capacitor and the D.C. voltage is applied across theenergizing coil.

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention, itself, however,both as to its organization and operation, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of one embodiment of the present invention;

FIG. 2 is a timing diagram depicting the circuit operation of FIG. 1;

FIG. 3 is a circuit diagram of another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a diagram of a relay drive circuit arrangement 10 inaccordance with the preferred embodiment of the present invention.Circuit 10 has an input stage having a first transistor 24 and an outputstage having a second transistor 48 which is used to control theactuation of an electromagnetic relay 50. Transistors 24 and 48 areshown as being of a NPN type transistor. However, it should berecognized that other types of transistors, such as Field EffectTransistors (FET) may also be used if desired. Transistors 24 and 48respectively function as a first and second switching means having afirst, a second, and a third electrode, which for a typical transistorare the emitter, the base, and the collector electrodes respectively.Similarly, electromagnetic relay 50 is shown as having a single pair ofnormally-open contacts 52 which become closed upon the energization ofrelay 50 to interconnect terminals 56 and 58. However, it should also berecognized that electromagnetic relay 50 may have other normally-open ornormally-closed contacts that may be utilized for additional controlfunctions.

In general, relay drive circuit 10 operates upon an occurrence of anelectrical signal 20 to actuate relay 50, which, in turn, closes itscontacts 52 interconnecting terminal 56 to terminal 58. The electricalsignal 20 is typically transmitted to drive circuit 10 as a commandsignal to notify an external source (not shown), via the interconnectionof terminals 56 and 58, that an alarm condition has been detected.

The drive circuit 10 is connected to a power supply (not shown), havinga positive voltage potential V_(BB) + and a negative voltage potentialV_(BB) -, via terminals 12 and 14 respectively. The voltage potentialsV_(BB) + and V_(BB) - are conducted to the components of drive circuit10 via busses 16 and 18, respectively, as shown in FIG. 1.

The base of transistor 24 of drive circuit 10 is coupled to the appliedelectrical signal 20 via a conductor 22. The emitter of transistor 24 isconnected to the buss 18 via a conductor 28. The collector of transistor24 is connected to the cathodes of a pair of diodes 30 and 32 viaconductor 26. The anode of diode 30 is connected to one side of a firstresistor 34. The anode of diode 32 is connected, via conductor 40, to afirst side of a parallel arrangement comprising an energizing coil 51 ofthe relay 50 and a diode 54. The second side of parallel arranged coil51 and diode 54 is connected to the buss 16.

The collector of the second transistor 48 is also connected to the firstside of the parallel arranged coil 51 and diode 54 via conductor 44.Transistor 48 has its base connected to the buss 18 via a secondresistor 46. The emitter of transistor 48 is connected to one side of acapacitor 36 and also to the anode of a first diode 38 which is furtherconnected to buss 18.

Capacitor 36, diode 38 and first resistor 34 are connected to form aserially-arranged resistor-capacitor-diode network 35, which as shown inFIG. 1, is connected across busses 16 and 18. This serially-arrangednetwork 35 provides for the capacitor 36 to be charged to a potentialsubstantially equal to the voltage difference between V_(BB) + andV_(BB) -. The charged capacitor 36 provides for two (2) functions; (1)it controls the conductive state of transistor 48, and (2) it dischargesthe charge stored across capacitor 36 for the actuation of relay coil 51upon the occurrence of the applied electrical signal 20. The result ofcharging network 35 is that the energizing coil 51 is energized with twolevels of voltages, (1) a first relatively high voltage level foractuating or picking-up relay 50 to place it in its operate state and(2) a second voltage level which is lower than the first for maintainingor holding-in the operate state of relay 50.

Reference is now made to the timing diagram of FIG. 2 which shows thesequence of the operation of the drive circuit 10. The operation ofcircuit 10 is controlled by the presence and absence of the appliedsignal 20. In the absence of signal 20 the initial condition of thecapacitor 36 is that it is charged to a voltage V_(c) having a valuesubstantially equal to the voltage potential between (V_(BB) + andV_(BB) -). Also in the absence of signal 20, further initial conditionsof circuit 10 are, (1) transistor 24 is nonconductive or OFF, (2)transistor 48 is non-conductive or OFF, (3) diodes 30, 32, and 54 arenon-conductive or OFF, (4) the voltage across relay coil 51, that isV₅₁, is zero and (5) relay 50 is its non-operate state and its contacts52 are opened.

Upon the presence or occurrence of signal 20 the following events areinitiated, (1) signal 20 renders transistor 24 conductive, (2) theconduction of transistor 24 provides a path to render diode 30conductive, (3) the conduction of diode 30 provides a path to initiate adischarge of capacitor 36 which is charged to the voltage Vc, (4) theinitiation of the discharge of the voltage Vc renders diode 38non-conductive and conversely transistor 48 conductive, and (5)conduction of transistor 48 provides a path to couple the energizingcoil 51 across a voltage potential which is the summation of thepotential difference between (V_(BB) + and V_(BB) -) and Vc. Theinternally developed voltage Vc is effectively a temporary serialbattery source that becomes additive with the external power source(V_(BB) + and V_(BB) -) upon conduction of transistors 24 and 48. Thetemporary and external sources become coupled across the energizing coil51 with one end of coil 51 becoming coupled to the negative potential ofcapacitor 36 and the other end of the coil being connected to theV_(BB) + potential. The total voltage potential applied acrossenergizing coil 51 is substantially the combined voltage potential ofthe temporary and external power sources which is the summation of Vcand V_(BB) + to V_(BB) -. Reference is now made back to FIG. 1 tofurther describe the interconnecting paths caused by the occurrence ofsignal 20.

The cathode of diode 30 becomes coupled to the V_(BB) - potential (buss18) upon the conduction of transistor 24 which in turn causes aforward-bias of diode 30 andcouple the positive terminal of capacitor 36to buss 18 and initiates the discharge of Vc. The coupling of Vc to buss18 provides a potential to (1) the cathode of diode 38 to render itnon-conductive and (2) to the base of transistor 48 to render itconductive. Conduction of transistor 48 provides a path for thesummation of the potential difference between V_(BB) + and V_(BB) - andVc to be connected across relay coil 51. The result of the conduction oftransistor 48 is that the voltage applied across coil 51 (V₅₁) issubstantially equal to the sum of Vc and the potential differencebetween V_(BB) + and V_(BB) -. The initial voltage V₅₁ is thereforesubstantially equal to twice the potential difference V_(BB) + andV_(BB) -. The voltage V₅₁ is dependent upon the voltage Vc which decaysin accordance with the circuit parameters of the inductive and resistiveelements of coil 51 and the capacitance of capacitor 36. b 2) holdingcurrent for relay coil 51 flows from V_(BB) +, through coil 51, throughdiode 32, and transistor 24 to V_(BB) -. Diode 30 remains conductivebecause of the current of resistor 34, whereas transistor 48 becomesnon-conductive because the voltage potential supplied by Vc whichinitially rendered transistor 48 conductive decreased to a value whichwill not sustain conduction. Reference is now made back to FIG. 2 tocomplete the description of the sequence of operation of the drivecircuit 10.

The pick-up time of relay 50 is shown as a time t which is initiatedupon the occurrence of electrical signal 20 and terminated before themid-point, shown as 55, of the voltage decay of Vc. The termination ofthe pick-up time t is shown in FIG. 2 as occurring at a value equal toabout one-half of the mid-point 55. It should be noted that the pick-upt for relay 50 is dependent on the amplitude of the voltage V₅₁. Thehigher the amplitude of V₅₁ the less the pick-up time of relay 50. Forexample, the pick-up time of relay 50 may be reduced to two thirds (2/3)of its initial value if the amplitude of V₅₁ is increased by a factor oftwo (2).

Upon the decay of Vc to its minimum value, the voltage V₅₁ has a valuesubstantially equal to the voltage potential difference between V_(BB) +and V_(BB) -. The voltage potential between V_(BB) + and V_(BB) - istypically chosen to be equal to or be greater than the voltage requiredto pick-up relay 50. Relay 50 continues to be energized for theremainder of the time period during the presence of input signal 20.Upon the termination of input signal 20 five (5) events occur; (1)transistor 24 becomes nonconductive, (2) diode 38 becomes conductive,(3) diode 30 becomes nonconductive, (4) diode 32 becomes nonconductive,and (5) the potential across relay coil 51 suddenly reverses because itsinductive current not must flow through diode 54. The conduction ofdiode 38 completes a path of couple the charging network 35 across thebusses 16 and 18 and produces a voltage potential rise across capacitor36 (Vc) shown in FIG. 2. The charging rate of capacitor 36 is dependentupon the time-constant of network 35 which is determined by the valuesselected for resistor 34 and capacitor 36.

The voltage V₅₁ decreases to a value below its initial value upon theremoval of signal 20 because of the electromagnetic relay 50 having aninductive type current flowing through its coil 51, begins to dischargeits stored energy. The inductive current flowing through diode 54provides a path to discharge the stored energy in the coil 51.Conduction of diode 54 allows the coil 51 to deenergize and thus allowthe relay contacts 52 to open. The time required for the inductivecurrent to decay and the contacts to operate is commonly known as thedrop-out time, and is shown in FIG. 2 as 20 t. The usage of the term "20t" is meant to represent that the discharge time of relay 50 isapproximately 20 times its pick-up time t. The drive circuit 10 remainsin this condition until the reoccurrence of the electrical signal 20.

It should now be appreciated that drive circuit 10 provides an actuatingelectrical signal V₅₁ for the electromagnetic relay 50 having twoamplitude levels, (1) an initial or first amplitude having a valuesubstantially equal to twice the voltage potential between V_(BB) + andV_(BB) -, for actuating relay 50 and (2) a reduced or second amplitudewhich is substantially equal to the voltage potential between V_(BB) +and V_(BB) - for maintaining actuation or holding-in of relay 50. Thefirst amplitude of V₅₁ may be chosen to greatly enhance the pick-up timeof relay 50 whereas the second amplitude of V₅₁ need only be sufficientto maintain actuation of relay 50.

The drive circuit 10, shown in FIG. 1, may be improved by a drivecircuit 60 shown in FIG. 3 depicting an alternate embodiment of thepresent invention. Drive circuit 60 is similar to drive circuit 10except for the following additions; drive circuit 60 has resistors 62and 64 and a diode 66 added as a bias arrangement for the emitter oftransistor 24, a zener diode 68 serially connected to diode 54, and aresistor 72 interconnected between diode 32 and energizing coil 51.

Resistor 62 and diode 66 provide a predetermined voltage at the emitterof the transistor 24. Resistor 64 is selected such that the current thatmay be present during the absence of signal 20 may develop a voltagepotential across resistor 64 that has a value slightly less than thevoltage potential across the forward conducting diode 66 to therebymaintain transistor 24 nonconductive during the absence of signal 20.

The zener diode 68 increases the voltage drop provided by a dischargepath formed by diode 54 and zener diode 68, which in turn decreases thetime required to decrease the stored inductive energy inherent in theenergized relay 50 and thus shortens the drop-out time of relay 50.Resistor 72 is serially added to diode 32 to reduce the holding currentfor energizing coil 51. Resistor 72 may be selected to adapt the holdingcurrent of drive circuit 60 to that which may be desired for relay 50.

It should now be appreciated that both drive circuits 10 and 60 eachhaving an internal charging network provide a means in which the speedof response of the electromagnetic relay is greatly enhanced withoutrequiring any increase to the voltage requirement of the power source ofa subsystem.

While I have shown and described particular embodiments of my invention,it will be obvious to those skilled in the art that various changes andmodifications may be made without departing from my invention in itsbroader aspects; and I, therefore, intend herein to cover all suchchanges and modifications as fall within the true spirit and scope of myinvention.

What is claimed is:
 1. A drive circuit responsive to an appliedelectrical signal for controlling the actuation of a relay having anenergizing coil with a first and a second end arranged across an outputstage of the circuit, said circuit being adapted to be energized by aD.C. voltage present between a positive and a negative potential, saidcircuit comprising;a charging circuit comprising a serially-arrangedfirst resistor, a capacitor, and a first diode, said first resistorhaving a first end connected to said positive potential, said firstdiode having its cathode connected to said negative potential, a secondand a third diode, said second and third diodes each having theircathodes connected together at a first end, said second diode having itsanode connected to a second end of said first resistor and to a firstend of said capacitor, said third diode ectrode being connected to saidnegative potential, said second electrode being adapted to receive saidelectrical applied signal, and said third electrode being coupled to theconnected cathodes of said second and third diodes, said first switchingmeans being arranged so that (i) when in its nonconductive state itallows said capacitor to be charged to a voltage substantially equal tosaid D.C. voltage and (ii) upon the occurrence of said appliedelectrical signal it is rendered conductive and supplies a path toforward-bias said second diode and initiate a discharge of saidcapacitor; a second switching means having first, second and thirdelectrodes, said first electrode being connected to a second end of saidcapacitor, said second electrode being coupled by a second resistor tosaid negative said energizing coil for dissipating inductive storedenergy; said charging circuit, said second and third diodes, and saidfirst and second switching means being arranged so that upon theoccurrence of said applied electrical signal, both of said first andsecond switching means are rendered conductive and an initial voltageequal to the sum of the charged potential of said capacitor and saidD.C. voltage is applied across said energizing coil to actuate saidrelay.
 2. A circuit according to claim 1 wherein said initial voltagedecreases upon discharge of the capacitor to a value substantially equalto said D.C. voltage which is sufficient to maintain actuation of saidrelay.
 3. A circuit according to claim 1 wherein said means fordissipating inductive stored energy of said energizing coil comprises afourth diode having its cathode connected to said first end of saidenergizing coil and its anode connected to said second end of saidenergizing coil, said fourth diode providing a conductive path fordissipating inductive stored energy which flows upon the termination ofsaid applied electrical signal.
 4. A relay drive circuit according toclaim 3 further including a zener diode having an anode and a cathode,said zener diode being serially connected with said fourth diode,wherein said serially connected zener diode and said fourth diodereduces the drop-out time of said relay.
 5. A relay drive circuitaccording to claim 1 further including a fourth diode, a third resistorand a fourth resistor;said fourth diode and said third resistor beingserially arranged across said D.C. voltage with the cathode and theanode of said fourth diode being connected respectively to said negativepotential and to said emitter electrode of said first transistor, saidfourth resistor connected between said base electrode of said firsttransistor and said negative potential, said fourth diode and said thirdand fourth resistors selected to provide a bias voltage between saidemitter and said base electrodes that is less than required to rendersaid first transistor conductive.
 6. A relay drive circuit according toclaim 1 further including a fifth resistor interconnected between saidfirst end of said energizing coil and said anode of said third diode,said fifth resistor reducing a holding current flowing through saidenergizing coil, whereby selection of the resistance value of said fifthresistor correspondingly adapts the holding current to that desired forsaid relay.
 7. A circuit according to claim 1 wherein said firstswitching means comprises a first transistor and wherein the firstelectrode is an emitter of the first transistor, the second electrode isthe base of the first transistor and the third electrode is thecollector of the first transistor, and wherein said second switchingmeans comprises a second transistor and wherein the first electrode isan emitter of the second transistor, the second electrode is the base ofthe second transistor and the third electrode is the collector of thesecond transistor.